Efflux Pump-Mediated Quinolone Resistance in Staphylococcus aureus Strains Wild Type for gyrA , gyrB , grlA , and norA

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Abstract
Fluoroquinolone efflux was studied in 47 Staphylococcus aureus clinical strains with MICs of ciprofloxacin (CFX) of ≤2 μg/ml. Forty-three strains were wild type for gyrA,gyrB, and grlA quinolone resistance-determining regions and for norA and its promoter region. Forty of these strains (MICs of CFX, 0.1 to 0.2 μg/ml) did not show efflux of fluoroquinolones. Three strains (MICs of CFX, 1 to 2 μg/ml) showed efflux. These results suggest that efflux can appear in S. aureus clinical strains in the absence of mutations innorA and its promoter.